[MM-37772] Idiomatic naming (URL, URI, API) (#18128)

* s/Url/URL/g & s/Uri/URI/g

* s/Api/API/g

app/webhook_test.go

@@ -611,11 +611,11 @@ func TestTriggerOutGoingWebhookWithUsernameAndIconURL(t *testing.T) {
 		WebhookResponse            *model.OutgoingWebhookResponse
 	}
 
-	createOutgoingWebhook := func(channel *model.Channel, testCallBackUrl string, th *TestHelper) (*model.OutgoingWebhook, *model.AppError) {
+	createOutgoingWebhook := func(channel *model.Channel, testCallBackURL string, th *TestHelper) (*model.OutgoingWebhook, *model.AppError) {
 		outgoingWebhook := model.OutgoingWebhook{
 			ChannelId:    channel.Id,
 			TeamId:       channel.TeamId,
-			CallbackURLs: []string{testCallBackUrl},
+			CallbackURLs: []string{testCallBackURL},
 			Username:     "some-user-name",
 			IconURL:      "http://some-icon/",
 			DisplayName:  "some-display-name",